Although biomechanical and psychosocial variables have been identified as independent risk factors in the development of musculoskeletal injury, physiologic mechanisms that underlie potential interactions among these risk factors remain poorly understood. Monoaminergic inputs to spinal motor neurons are correlated with levels of physiologic arousal and generate persistent inward currents (PICs) that function to amplify existing sources of excitatory synaptic input to the motor neuron pool. These findings suggest a novel mechanism of stress-related musculoskeletal injury in which heightened physiologic arousal is proposed to cause an increase in tonic excitability of spinal motor neurons that is mediated by the enhanced activation of persistent inward currents. Three specific aims will test this hypothesis using the paired motor unit analysis technique to estimate the magnitude of persistent inward currents in human subjects by comparing the discharge rates of a low threshold motor unit (control unit) measured at the time of recruitment and derecruitment of a slightly higher threshold motor unit (test unit). Aim 1 will compare the contribution of persistent inward currents to single motor unit activity during voluntary contractions of the upper trapezius muscle when subjects are exposed to low and high levels of psychosocial stress. To determine whether persistent inward currents contribute to patterns of muscle activation that have been shown to increase the risk of musculoskeletal injury, Aim 2 will determine the association between PIC estimates and sustained motor unit activity during intermittent rest breaks following exposure to low and high levels of psychosocial stress. Based on findings that some individuals are more susceptible to the negative effects of psychosocial stress on neuromuscular function, Aim 3 will determine the effect of individual differences in trait and state anxiety on stress-induced physiologic arousal and PIC contributions to motor unit activity. Findings from this research will provide the first evidence to support a functional role for intrinsic mechanisms in regulating the tonic excitability of spinal motor neurons with changes in arousal in human subjects. [unreadable] [unreadable] [unreadable]